When transporting natural gas, the most efficient means is to transport it in a liquid state. Liquefied natural gas (“LNG”) takes up only a fraction (about 1/600) of the volume of natural gas in its gaseous state, and is transported by tanker ships or trains equipped with cryogenic compartments. LNG is stored in cryogenic compartments either at or slightly above atmospheric pressure. To produce LNG, natural gas is cooled below its boiling point (about −161° C. at ambient pressure). While it is practical to transport LNG because it takes up a fraction of the volume of natural gas in its gaseous state, natural gas is usually required in its gaseous state for combustion. LNG may be converted into its gaseous form by raising the temperature of the LNG.
Converting natural gas to and from its liquid and gaseous states at a desired location is critical for efficient transportation and use. For instance, transporting natural gas as LNG to a location for on-site use and converting LNG into compressed natural gas (“CNG”) on-site is preferred over transporting natural gas in its bulky, gaseous form. Similarly, CNG may be preferably converted into LNG at a reclamation site for transportation to another location. While a portable system for converting CNG into LNG is preferred, the conversion requires a substantial amount of energy, limiting the portability of many systems to locations with access to a utility grid. Further, it is critical to reduce the amount of energy used during conversion of natural gas into LNG, particularly for small-batch conversion, because a substantial amount of energy is required.
One attempt at converting natural gas into LNG efficiently is described in U.S. Pat. No. 7,071,236 (“the '236 patent”) issued to Fischer et al. on Jul. 4, 2006. In particular, the '236 patent describes a method for liquefying natural gas by cooling, expanding, and separating the natural gas. The natural gas is chilled by a first heat exchanger, which is primarily cooled by a closed-loop cooling system. The chilled gas is expanded in a first turbine to produce a first liquid fraction and a first gas fraction, and directed to a first separator. The first liquid fraction collects at the bottom of the first separator and is released to a second heat exchanger. After being chilled in the second heat exchanger, the chilled first liquid fraction is expanded in a second turbine to produce a second liquid fraction and a second gas fraction, and directed to a second separator. The second liquid fraction collects at the bottom of the second separator and is released from the system as LNG. The second gas fraction collects at the top of the second separator and serves as a coolant for the second heat exchanger before being directed to a compressor. Meanwhile, the first gas fraction collects at the top of the first separator and serves as a coolant for the first heat exchanger, in addition to the closed-loop cooling system, before being directed to the compressor. The compressor compresses the first and second gas fractions and directs the compressed gas to a Fischer-Tropsch unit (“FT unit”).
The FT unit provides, at least partly, the energy required for the recompression of coolants used in the closed-loop cooling system. The process performed in the FT unit is exothermic and the heat produced during the reaction is used to produce steam. The steam is expanded in turbines that drive the compressors used to compress the coolants in the closed-loop cooling system.
Although the method of the '236 patent may help convert natural gas into LNG, the method may be limited. That is, the method of the '236 patent requires an FT unit that relies on steam to produce power. This complicates the system because a water supply must be maintained apart from the supplies in the system. Further, the system requires additional components, such as the FT unit, which complicates the system and limits portability of the system based on equipment size and availability and system requirements. As the FT unit only drives the compressors that compress coolant in the closed-loop cooling system, the system may still require a utility grid or other energy source to drive the compressors outside of those powered by the FT unit.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or elsewhere in the prior art.